1. Field of the Invention:
This is a continuation of application Ser. No. 719,681, filed Apr. 4, 1985 now abandoned.
This invention relates to automatic focusing devices, and more particularly to a device for automatically adjusting the position of a focusing lens member in a zoon lens as the in-focus position for the same object varies with variation of the focal length of the entire system.
2. Description of the Prior Art:
Known zom lenses, such as the rear focus type, have focusing members whose in-focus position varies as the focal length changes although the object remains at the same distance. In such zoom lenses, the in-focus position of the member focusing on the object depends on the present value of the focal length. Hence, if one zooms after the lens has been set in sharp focus, the image plane for the object shifts. This will be explained by using FIG. 1.
FIG. 1 shows the relationship, in a rear focus type zoom lens, between the focal length and the distance a focusing member moves from its infinity position, i.e. with an object at infinity, for a number of discrete values of object distance. Here, the ordinate is the focal length, and the abscissa is the amount the focusing member moves. As is obvious from the curves of this graph in FIG. 1, a zoon lens of the rear focus type exhibits a common characteristic for all object distances except infinity. Specifically, the in-focus position or the movement of the focusing member varies as a quadratic function of the focal length with an object at a particular distance. In other words, while the focusing member is held stationary during zooming, the image plane shifts so that the image sharpness on the film plane decreases. Also, the amount the focusing member is required to move for the same change of focal length increases as the object distance decreases. For example, in the zoon lens of the rear focus type specified by FIG. 1, to focus from infinity to an object distance of about 2.5 meters at a wide angle end and bring the plane of a sharp image of the object into coincidence with the film plane in the camera requires the focusing member to move axially only 1.7 mm or thereabout. But at the telephoto end the focusing lens member must be moved forward by a distance as long as about 10 mm.
Such problems of the zoon lens may be alleviated by relying on the action of an automatic focusing device that responds to detection of the movement of the image plane to move the focusing lens to an in-focus position, as, for example, disclosed in U.S. Pat. No. 3,972,056. But this permits compensation for the image shift to follow zooming only when the object is bright enough to allow the image sharpness detector to respond rapidly, or when the zoom speed is slow. If the object is dim, or the zoom actuator is operated quickly, the automatic focusing device is unable to catch up to the movement of the image plane and compensate for it.
In such a case, the object image on the film plane is confused enough so that the image sharpness detector becomes unable to carry out a normal focus detecting operation. Therefore, the automatic focusing device is switched to a so-called "search" mode in which the focusing lens member is first moved arbitrarily in the forward or backward directions. If this selected direction coincides with that in which the focusing lens member approaches the in-focus position, it will take the minimum time to establish the in-focus condition. But when it coincides with the opposite direction, a far longer time is needed for the focusing lens member to reach the in-focus position. This increases the possibility that quick actuation of the zoom ring will cause a photographer to miss a good opportunity to snap a picture.